A spot film device is a combination radiographic filmer and fluoroscopic imager. Such devices are commonly mounted for use in conjunction with an x-ray table.
The spot film device includes a housing, sometimes called a "tunnel", which is movably supported on the xray table by a movable column or tower. The tower supports the spot film tunnel component for movement longitudinally and laterally with respect to the table top in vertically adjustable planes that are parallel to the table top. An x-ray source is located within the table body and is mechanically coupled to move in unison with the longitudinal and lateral motion of the spot film device. The source propagates x-rays from within the table body, upwardly through the table top and through the body of a patient when positioned on the table top. The pattern of radiation emergent from the upper surface of the patient's body defines an image of the patient's anatomy.
The rear portion of the tunnel nearest the tower, defines a parking space or position for a radiographic film holding cassette which is used in connection with the radiographic filming function of the spot film device. Transport means is provided for mounting a film cassette for motion between the parking position and an active filming, or "expose", position, more distant from the fluoro screen or image amplifier. The transport mechanism includes manual, or electromechanical servo, power means, the latter of which, on command, projects the film cassette bearing a portion of radiographic film into the x-ray path expose position to take radiographs of selected images that are detected in the fluoroscopic mode. After each radiograph is taken, the cassette is returned to the park position, and the spot film device continues fluoro mode operation.
The general technology relating to spot filming devices is well known. The following U.S. patents, here expressly incorporated by reference, disclose details of typical spot film devices: Stava et al., U.S. Pat. Nos. 2,767,323; Stava et al., 2,749,445; Barrett et al., 3,173,008; and Hunt, 4,357,538.
The spot film device thus facilitates selected clinical observations made during fluoroscopy to be permanently recorded on film. The speed of cassette transport between these two positions is of great clinical importance, due to involuntary physiological movement within the patient's body, such as peristalsis. It is important that this transport time be as short as possible to permit accurate representation on film without blurring or altogether missing an observation. While striving for minimal transport time, it is nonetheless necessary to also minimize, or avoid altogether, vibration of the structure supporting the spot film device, which vibration is caused by the quick transport of the film cassette between the park and expose positions. This structural vibration, if uncontrolled, can cause blurred images on the film, diminishing their diagnostic usefulness.
Prior methods of cassette transport control include various forms of speed control which attempt to shorten transport time. Other proposals have included force buffering, which attempts to control structural vibration. All prior methods of cassette transport, however, fall short of providing an effective comprehensive solution because none address the complete problem of system dynamics, i.e., the synergistic interaction of the spot filmer and its associated support structure.
One technique for operating a cassette transport servomechanism is to abruptly apply to a servo drive motor of a spot film device a square wave voltage input and to abruptly terminate the square wave input application, such as by way of a limit switch, when the cassette approaches or reaches the end of its predetermined travel path. This technique, however, due to the abruptness of the application and removal of the voltage input, produces significant residual vibration in the structural components of the spot filmer system, such as in the support tower, which is unacceptable for reasons noted above. Therefore, the proposed square wave input method, sometimes called "bang-bang", fails to deal effectively with accommodation of control of the mechanical structural dynamics, either as an integral part of the control strategy or otherwise, to minimize structural motion and improve film image resolution and quality.
It has been proposed, in general industrial servo mechanism technology, to apply mechanical means, such as cams, to mechanical linkages for driving movable components, to reduce vibration attendant upon moving the component from one position to another along a predetermined travel path. Such techniques, however, address only the particular aspect of mechanical control, and do not address the aspect of input command signals to the servomechanism drive motor. Thus, such a purely mechanical approach is not entirely satisfactory in reducing vibration, inasmuch as it does not consider and accommodate the particular attributes of the total system.
In mechanical general industrial embodiments, mechanical means have been provided to impart a generally trapezoidal acceleration profile to motion of a movable component. In such a proposed servo system, the square wave voltage input was applied as described above.
In accordance with another proposal, a spot filmer cassette transport servomechanism was provided wherein the voltage input to the servo motor defines a triangular waveform. According to this proposal, the amplitude of the voltage waveform is modified as a function of the mass of the cassette to be transported, in order to maintain the total transit time for the cassette motion at a constant value, irrespective of the cassette mass. Vibration suppression is believed to have formed no part of this proposal.
In accordance with still another proposal, a servo mechanism was provided for spot filmer cassette transport which includes a number of interchangeable cams, each defining a different acceleration profile for cassette movement. The cam is selected depending upon the mass of the cassette to be transported. This proposal, however, does not address the impact of the input voltage profile on vibration, and thereby does not address the total system needs for reduction of residual vibration.
In the absence of effective cassette transport vibration suppression, designers have sometimes compensated by building such systems with highly rigid support structure, less susceptible to vibration. This measure, however, undesirably increases the weight and cost of systems of this nature.
A known spot film device, a model 1720A manufactured and sold by Picker International, of Cleveland, Ohio, U.S.A., employs a servo motor to provide film cassette motion. The motor control provides an input voltage to the motor which is a ramp function, in an attempt to reduce residual vibration. As is the case of the other attempts described above, this particular prior art attempt, while helpful to a degree, represents an incomplete solution to the vibration problem. The input voltage profile is not specifically tailored to the mechanical system.
Another prior art spot film device, designated model 1717, manufactured and sold by Picker International, Cleveland, Ohio, U.S.A., utilizes a rotating arm mechanism to provide film cassette motion. Mechanisms such as these, however, are mechanically complex and make optimizing motion forces difficult.
A general object of this invention is to provide a spot film device in which the electrical servo input, servo cassette transport system radiographic system structure, and the cassette itself are all mutually "tuned" to one another to reduce or eliminate residual vibration from cassette transport motion.